Before I get into the linkorama it's worth noting that you should be careful what licenses are attached to the different images. If you are producing something for sale, may I recommend you pop over and throw some fabric at a proper artist. Most places would like a link back, which is no real hardship for any free RPG, right?
- Artclipart.com. Have a big selection, most cartoon like but some more realistic ones too. The list is categorised into Castles, Dragons, Elves, Fairies, Fairy Tales, Medieval, Pirates, Unicorns and Wizards.
- Karen's Whimsy has a great selection of public domain images like the plates you might find in historical books.
- Medieval Woodcuts Clipart Collection is a fine collection of authentic (well they look authentic to me) black and white images CC where the owner would like a link back.
- WP Clip Art has a large, categorised collection of all sorts of images. I'd recommend the Fictional Characters and World History categories.
- Open Clip Art has a huge number of items. Might be a bit difficult to find what you need as they are grouped together in packs but the quality of those I randomly picked is high.
- Zorger.com is a great search tool for loads of CC images, many of them historical. Very cool.
- Abyssal Maw's CC images he posted up on the RPG Site. Nice!
- From old books has a load of scans from really old books. Obviously, this throws up lots of historical images. Well worth a look.
- Incredible Clip Art Images is an aptly named list of clip art sites.
- DMOZ Clip art list. DMOZ is the free open directory and there are easy-to-browse categories. Medieval and Renaissance seems like the most appropriate.
- Jupiter images has a list of other royalty free image sites. Be warned! Inbetween the free stuff is paid-for-stuff too.
If you're looking for a great tool for shaping these images for your own use, I can heartily recommend GIMP, which has radically improved in recent years to punch as hard as Photoshop.
Of course, if you are aware of any free (public domain or Creative Commons) images that might be relevant for your RPG, please pop the link in a comment!